JOURNAL OF VIROLOGY, Apr. 1992, p. 2031-20360022-538X/92/042031-06$02.00/0Copyright ©) 1992, American Society for Microbiology

Functional Comparison of the Basic Domains of the Tat Proteins ofHuman Immunodeficiency Virus Types 1 and 2 in trans Activation

B. ELANGOVAN, T. SUBRAMANIAN, AND G. CHINNADURAI*

Institute for Molecular Virology, St. Louis University School ofMedicine,3681 Park Avenue, St. Louis, Missouri 63110

Received 28 October 1991/Accepted 2 January 1992

The trans-activator Tat proteins coded by human immunodeficiency virus type 1 (HIV-1) and HIV-2 appearto be similar in structure and function. However, the Tat protein of HIV-2 (Tat2) activates the HIV-1 longterminal repeat (LTR) less efficiently than Tatl (M. Emerman, M. Guyader, L. Montagnier, D. Baltimore, andM. A. Muesing, EMBO J. 6:3755-3760, 1987). To determine the functional domain of Tat2 which contributesto this incomplete reciprocity, we have carried out domain substitution between Tatl and Tat2 by exchangingthe basic domains involved in Tat interaction with its target trans-activation-response (TAR) RNA structure.Our results indicate that Tatl proteins containing substitutions of either 8 or 14 amino acids of the basic domainof Tat2 exhibited reduced trans activation of the HIV-1 LTR by about 1/20 or one-fourth the level induced bywt Tatl. In contrast, Tat2 containing a substitution of the 9-amino-acid basic domain of Tatl trans activatedHIV-1 LTR like native Tatl. A substitution of the highly conserved core domain of Tat2 with that of Tatl didnot have any significant effect on trans activation of the H1V-i LTR. These results indicate that the basicdomain of Tat2 contributes to its inefficient trans activation of the HIV-1 LTR. Mutation of an acidic residue(Glu) located between the core domain and the Arg-rich basic domain of Tat2 at position 77 to a Gly residueincreased the activity of Tat2 substantially. These results further suggest that the presence of an acidic residue(Glu) adjacent to Arg-rich sequences may at least partially contribute to the reduced activity of the Tat2 basicdomain.

The human immunodeficiency viruses (HIVs) have beenetiologically linked with AIDS. These pathogenic humanretroviruses have been grouped under two types. HIV type1 (HIV-1) is the predominant type isolated from most pa-tients with AIDS. HIV-2, which was isolated from patientswith AIDS in western Africa (9, 20), is less prevalent.Further, HIV-2 also appears to be less virulent than HIV-1.The primary sequences of HIV-1 and HIV-2 vary substan-tially (25). HIV-2 is more closely related to simian immuno-deficiency viruses (SIVs) than to various strains of HIV-1. Inspite of the variations in the primary sequences of theirgenomes, HIV-1 and HIV-2 are similar in genetic organiza-tion. Particularly, the structure and function of variousregulatory genes are conserved among the various primateimmunodeficiency viruses (HIV-1, HIV-2, and SIV). Forexample, both HIV-1 and HIV-2 code for a strong transactivator, Tat, which activates the expression of geneslinked to the viral long terminal repeat (LTR). Although theprecise mechanism of Tat-mediated trans activation stillremains unclear, a number of recent studies with the Tatprotein of HIV-1 (referred to here as Tatl) indicate that Tatmay play an important role at the level of transcriptionalinitiation and elongation (reviewed in reference 11). Unlikemost of the eukaryotic transcriptional activators, Tat func-tions by direct interaction with an RNA target termed TAR(4, 17, 19, 28). In addition, Tat may also play a role intranslational utilization of viral mRNA under certain condi-tions (5). Although the biochemical properties of the Tatprotein of HIV-2 (referred to here as Tat2) have not beenextensively studied, it appears that Tat2 may function in amanner similar to that of Tatl. Both Tatl and Tat2 have anobligatory requirement for TAR in most of the cell types (1,

* Corresponding author.

3, 15, 19, 28) for trans activation of the LTR. In addition,both Tatl and Tat2 contain protein domains that are con-served (25) (see Fig. 1). Further, both Tatl and Tat2 exhibitreciprocal trans-activation effects on the HIV-1 and HIV-2LTRs.

In spite of the structural and functional relatedness of Tatland Tat2, it has been reported that Tatl and Tat2 efficientlytrans activate the HIV-2 LTR, while Tat2 is less efficientthan Tatl in activation of the HIV-1 LTR (15). These resultssuggest that Tatl and Tat2 may have some functional dif-ferences with regard to their action on the HIV-1 LTR. Onthe basis of secondary structure analysis and mutationalstudies, it has been suggested that the TAR structure ofHIV-2 (TAR2) contains two TAR elements (1, 3, 18, 30).These studies imply that Tat2 may function more efficient-ly on a duplicated stem-loop structure, as in TAR2, thanon a single stem-loop, as in the TAR of HIV-1 (TAR1). Insupport of this view, it has been reported that the additionof a second stem-loop to TAR1 increased the responsive-ness to Tat2 (3). However, it is not known whether anyspecific protein region of Tat2 is responsible for the differ-ential activity. A number of recent studies have revealedthat an arginine-rich basic domain of Tatl specificallybinds to the pyrimidine bulge region of the TAR1 structure(10, 29, 34). These binding studies have further established adirect correlation between Tat-binding to TAR and transactivation (13, 29). Since Tat appears to function by directinteraction with TAR RNA, it is possible that the basicdomain of Tat2 may be inefficient in recognizing TARL. Herewe report the construction of chimeric Tatl and Tat2 sub-stitution mutants expressing the basic domains of Tat2 andTatl, respectively, and show that the basic domain of Tat2contributes to the inefficient trans activation of the HIV-1LTR.

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2032 ELANGOVAN ET AL.

MATERIALS AND METHODS APlasmids and mutants. Plasmid pLTR-1 CAT, which ex-

presses the bacterial chloramphenicol acetyltransferase(CAT) gene under control of the HIV-1 (SF2) LTR, has beendescribed previously (27). Plasmid pTatl expressing the tatgene of HIV-1 (SF2) under transcriptional control of thecytomegalovirus immediate-early (CMV-IE) promoter in thevector pBC12 (24) was constructed by transferring theTat-coding sequences from a previously described plasmid,pTat (27). Plasmid pTat2 was constructed by cloning theTat2-coding genomic sequences (5783 to 8569) frompROD214 (15) into a pBC12-based vector.Mutants Tatl/2BD-A and Tatl/2BD-B were constructed

by cloning the corresponding double-stranded oligonucleo-tides coding for the Tat2 basic domain sequences betweenthe unique SaclI and Sacl sites of Tat57-A (22) and sub-sequently inserting the chimeric gene into the CMV-IE(BC12) expression vector. Mutant Tat2(99) was constructedby deleting the second exon region (sequences locatedbetween three PvuII sites) of Tat2. Mutations Tat2(99)/lBD,Tat2(99)77, and Tat2(99)/lCD were introduced by a triple-primer polynucleotide chain reaction method (22). The DNAfragments obtained from the polynucleotide chain reactionwere digested with SacI and were cloned between a SacI anda unique PvuII site in plasmid pTat2(99). Tat2(99)/lBDcontains a substitution of the basic domain of Tatl (resi-dues 48 to 57) for the corresponding Tat2 domain, locatedbetween residues 76 and 91. Mutant Tat2(99)77 containsa substitution of a glycine residue for a glutamic acid resi-due at position 77 of Tat2. Tat2(99)/lCD contains a substi-tution of the core domain of Tatl (residues 39 to 46) for thqcorresponding Tat2 domain, located between residues 67and 76.CAT assays. The functional activity of the various chi-

meric tat genes was tested by cotransfection of thepLTR-1CAT plasmid and the various Tat-expressing plasmids inHeLa or COS-7 cells. Cells (1.5 x 106 to 2 x 106/60-mm2dish) were transfected with 1,ug of pLTR-1 CAT and variousconcentrations of the Tat-expressing plasmids by the cal-cium phosphate method. All transfections included 1,ug ofpRSV 1-Gal, which expresses the Escherichia coli lacZ geneunder the transcriptional control of the Rous sarcoma virusLTR as an internal control for monitoring transfection effi-ciency. CAT activities were quantified by scraping theacetylated chloramphenicol spots and counting the radioac-tivity by scintillation counting. CAT activities are expressedrelative to the basal level of LTR-CAT expression.

Immunoprecipitation and indirect immunofluorescence.COS-7 cells (1.5 x 106 to 2.0 x 106/60-mm2 dish) were

transfected with 1 to 2p,g of various Tat-expressing plas-mids. A total of 48 h after transfection, cells were labelledwith 200 pCi of [35S]cysteine for 1 h. Immunoprecipitation ofTatl proteins was carried out with polyclonal rat antibodiesdirected against Tatl (2) or with rabbit antipeptide antibodyraised against a synthetic peptide corresponding to N-termi-nal 17 amino acids (26). Immunoprecipitation of Tat2 pro-

teins was carried out with rabbit polyclonal antibodies raisedagainst a synthetic peptide corresponding to amino acids 76to 99 of Tat2 (18). Indirect immunofluorescence analyses ofTatl and Tat2 proteins in COS-7 cells transfected withvarious plasmids were carried out essentially as describedpreviously (33). The Tat antibodies were used at a dilution of1:200, while the second antibody (fluorescein isothiocy-anate-conjugated goat anti-rabbit immunoglobulin G) was

used at a dilution of 1:50.

Tatl; Tatl/2BD

Sac 11 (48) Sac (57)

.:.:..--------- .~......I: ..........::::....

1BD GRKKRRQRRR2BD-A ERKGRRRR2BD-B ERKGRRRRTPKKTK

B

@-.YPtw.1oI.,-11 '!w "I

'5D: a

co m

FIG. 1. (A) Organization of Tatl and Tatl/Tat2 chimeric pro-teins. Domain map of Tatl (strain SF2) is based on previousmutational analysis (23). Tatl/2BD-A and Tatl/2BD-B contain sub-stitutions of the indicated residues of the basic domain from Tat2(ROD214). Mutants tatl/2BD-A and tatl/2BD-B were constructedby cloning the corresponding double-stranded oligonucleotides be-tween the unique SacII and Sacl sites of mutant tat57-A asdescribed previously (32). (B) trans activation of HIV-1 LTR byTatl/2BD mutants. HeLa cells were transfected with pLTR-1 CATalone or along with various Tat-expressing plasmids. CAT activitieswere determined 48 h after transfection as described in Materialsand Methods and are expressed relative to the basal level ofLTR-CAT expression.

RESULTS

Effect of Tat2 basic domain on trans activation by Tatl. Todetermine whether the basic domain of Tat2 contributes tothe lower level of trans activation of the HIV-1LTR, weconstructed two different substitution mutants (Fig.1A) ofthe tatl gene by substituting the Tatl basic domain (referredto here as 1BD) with the basic domain of Tat2 (2BD). Mutanttatl/2BD-A contains a substitution of an 8-amino-acid region(ERKGRRRR) of Tat2 in place of the 1BD (RKKRRQRRR).Mutant tatl/2BD-B expresses an additional 6-amino-acidregion of the 2BD (ERKGRRRRTPKKTK). The ability ofthese mutants to trans activate the expression of the reporterbacterial CAT gene linked to HIV-1 LTR (pLTR-1 CAT) wasdetermined by DNA transfection on HeLa cells (Fig.1B). Asexpected from previous studies (3, 15, 18), Tat2 (wild type[wt]) trans activated pLTR-1 CAT expression at aboutone-half the efficiency of Tatl. In contrast, mutant tatl!2BD-A, which expresses the arginine-rich half of the 2BD,trans activated pLTR-1 CAT poorly (6% of the Tatl-induced

J. VIROL.

UD CD Cl) r,0 N

-dalbi..

.-Aw., im,

WI

BASIC DOMAINS OF THE Tat PROTEINS OF HIV-1 AND HIV-2 2033

An 0m XN N

2i

29

B

o;4-W

A0m:_IenSm

Tat2; Tat(99)/11BD

,........................................................ ............ ............ .........

FIG. 2. Immunoprecipitation of Tatl and Tat2 proteins. COS-7cells (1.5 x 105/60-mm2 dish) were transfected with 1 to 2 ,ug ofvarious Tat-expressing plasmids. A total of 48 h after transfection,cells were labelled for 1 h with 200 ,uCi of [35S]cysteine. Immuno-precipitations of Tatl proteins were carried out with polyclonal ratantibodies directed against Tatl (2) or with rabbit antipeptideantiserum raised against a synthetic peptide corresponding to theN-terminal 17 amino acids (26). Immunoprecipitations of Tat2proteins were carried out with rabbit polyclonal antiserum raisedagainst a synthetic peptide corresponding to amino acids 76 to 99 ofTat2 (18). (A) Tatl and Tatl/2BD; (B) Tat2 and Tat2/lBD. Mindicates protein molecular weight markers. In panel B, Tat2(99)i1BD was precipitated with a mixture of Tatl and Tat2 antibodies.The bracketed regions contain Tat-specific bands. It appears thatboth antibody preparations do not have significant reactivity againstepitopes of the Tat2/1BD chimeric protein.

level). Inclusion of the Lys-rich half of the 2BD (tatl/2BD-B)further increased the level of trans activation to about 25%of the level observed with Tatl. Similar relative effects ontrans activation were also observed in COS-7 cells (resultsnot shown). These low levels of trans activation by thechimeric tat genes do not appear to be due to any defect inthe level of accumulation of the chimeric proteins, sinceCOS-7 cells transfected with mutants tatl/2BD-A and tatl!2BD-B expressed Tat protein at levels comparable to Tatl(Fig. 2A). Further, the low level of trans activation appearsto be primarily contributed to by the Tat2 basic domain,since we have previously shown that a heterologous basicdomain from HIV-1 Rev can efficiently substitute for theTatl basic domain when inserted at the same site within theTatl coding region (33).

Effect of Tatl basic domain on trans activation by Tat2. Tofurther confirm the partial functional reciprocity of the Tat2basic domain, we constructed a substitution mutant of Tat2containing a substitution of the 1BD (GRKKRRQRRR) (Fig.3A) and determined its activity on LTR-1 CAT expression.The sequences coding for the 1BD were inserted in frameinto the Tat2-coding sequences [exon 1; Tat2(99)] by poly-nucleotide chain reaction (22). For this chimeric gene con-

struction, we used a tat2 gene expressing only the first exon[tat2(99)] as the backbone. The trans-activation potential ofTat2(99)/lBD was compared with those of the parentalTat2(99), Tat2 (wt), and Tatl (Fig. 3B). The parental Tat2(99)activated LTR-1 CAT at about one-fourth the efficiency ofTatl and about one-half the efficiency of Tat2 (wt). How-ever, mutant tat2(99)/lBD trans activated LTR-1 CAT to alevel similar to that by wt Tatl, suggesting that the basicdomain is the primary determinant for the differential activ-ity of Tat2 on HIV-1 LTR. Since this Tat2 substitutionmutant exhibits near wt Tatl activity, all the protein regionsof Tat2 other than the basic domain may be functionallyequivalent to the corresponding Tatl regions.To ascertain that the enhanced trans-activation potential

observed with mutant Tat2(99)/lBD is indeed contributed to

2BD ERKGRRRRTPKKTK1BD GRKKRRQRRR

B0 4 N 0

9.

**I0

v

aa n

_ _

_ N S tS

i v

FIG. 3. (A) Organization of Tat2 and Tat2/Tatl chimeric pro-teins. Domain map of Tat2 is based on sequence similarity to Tatl,shown in Fig. 1A. Mutant tat2(99) was constructed by deleting thesecond exon region (sequences located between three PvuII sites) ofTat2. Mutant tat2(99)/lBD contains a substitution of the basicdomain of Tatl (residues 48 to 57) for the corresponding Tat2domain, located between residues 76 and 91. All Tatl and Tat2sequences are expressed from a CMV-IE-based expression vector,pBC12 (24). (B) trans activation (CAT assay) of HIV-1 LTR byTatl/2BD mutants. HeLa cells were transfected with 1 ,ug ofpLTR-1 CAT (27) alone or along with 0.1 ,ug of various Tat-expressing plasmids. CAT activities are as described in the legend toFig. 1.

by the 1BD sequences, we also constructed a differentsubstitution mutant [Tat2(99)/lCD] in which the core domain(domain C) of Tatl was substituted for the correspondingTat2 domain (Fig. 4A). Domain C is not only conservedamong the Tat proteins encoded by the various primateimmunodeficiency viruses, but it is also conserved in the Tatprotein encoded by a more distantly related equine infec-tious anemia virus (8, 14). Comparison of the trans-activa-tion potential of mutant Tat2(99)/lCD with that of theparental Tat2(99) indicates that the core domain of Tatl doesnot enhance the activity of Tat2 (Fig. 4B).The above results strongly suggest that the differential

activities of Tatl and Tat2 are primarily contributed to by thebasic domains of the respective Tat proteins. However,comparison of the sequences of the 1BD and the 2BDindicate that these domains share a substantial homology(Fig. 5, boxed areas). Further, the 1BD and the 2BD have asimilar net charge density (+8). However, an acidic residue

14.3

PV ll l (99)

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2034 ELANGOVAN ET AL.

A'I'a t 2(99) / I C D; Tat2 (99) 77

i iG I1cl (2_ t9) 7

2CD PLNKGLGrCY1CCD PRT R\KGLGISY

ization of Tatl protein. To determine whether the basicdomain of Tatl or Tat2 has any differential effect on subcel-lular localization of Tat proteins, COS-7 cells were trans-fected with the various Tat-expressing plasmids describedabove [Tatl, Tat2, Tat2(99), Tat(99)/lBD, Tat(99)77, Tat(99)/1CD] and expression of the various Tat proteins was deter-mined by indirect immunofluorescence with antibodies spe-cific for Tatl or Tat2 (Fig. 6). These studies indicate thatboth the wt Tatl and the wt Tat2 as well as the variousmutant Tat proteins efficiently localized in the nuclear andnucleolar regions. No apparent differences among the sub-cellular localization phenotypes of the various mutant pro-teins were observed.

I'at) (99). I C I)

N 0tt.

NE...e N*_ :

4

07i-

n a

m cc-o X X

_z C1 N1 N Na a %

FIG. 4. (A) Organization of Tat2(99) and mutants; (B) transactivation (CAT assay) of HIV-1 LTR by Tat2(99)/lCD orTat2(99)77. CAT activities are as described in the legend to Fig. 1.

(Glu) is present at the junction of the core Tat2 domain(domain C in Fig. 4A) and the basic domain (domain D inFig. 4A). To determine whether this acidic residue has aninhibitory role in the activity of the 2BD, we constructed amutant, Tat2(99)77, in which this Glu residue (position 77)was converted to a Gly residue. A Gly residue is also presentat the corresponding position in Tatl. The assay for thetrans-activation potential of this mutant (Fig. 4B) indicatesthat it trans activated HIV-1 LTR CAT expression to a levelmore or less similar to that of Tatl. This result furtherstrengthens the data presented above, which show thatdomain D of Tat2 contributes at least partially to therelatively inefficient trans activation of the HIV-1 LTR byTat2.

Effect of 1BD and 2BD on subcellular localization of Tat. Anumber of previous studies (21, 31, 33) have shown that thebasic domain is required for efficient nuclear-nucleolar local-

HIV-1 G R K K R R Q R R

HIV-2 E R K G R R R T P K K T K

FIG. 5. Comparison of the basic domains of Tatl and Tat2.

DISCUSSION

Our results demonstrate that the differential activities ofTatl and Tat2 on the HIV-1 LTR are primarily contributedto by the differences in the individual basic domains. Thisconclusion is based on our results obtained with the Tat2substitution mutants expressing the 1BD as well as with Tatlsubstitution mutants expressing the 2BD. In these studies,the increase or decrease in trans activation of the HIV-1LTR by various mutants clearly reflected the basic domaincontained. In contrast to the results obtained with the BDmutants, swapping of the conserved core domain (domain C)between Tatl and Tat2 did not alter the trans-activationpotential significantly. However, we cannot fully rule outany potential effects of other regions, such as the Cys-richdomain, in the differential trans-activation property, sincewe have not been able to construct a functional Tatl/Tat2chimera by exchanging the Cys-rich domain. Our conclusionthat the 2BD contributes to inefficient trans activation of theHIV-1 LTR by Tat2 is strongly supported by the single-amino-acid substitution mutant Tat(99)77, which exhibits en-hanced activity compared with Tat(99). It appears that thedifferential effects observed with various mutants may not beattributed to other characteristics of mutant proteins, suchas subcellular accumulation and transport to the nuclear-nucleolar locations.

Since it appears that the basic domain specifically binds toTAR, it is possible that the 1BD and the 2BD may havevaried affinity for the TAR1 structure, thus contributing tothe partial reciprocal activity. By functional substitutionwith a number of heterologous basic domains, we havepostulated that an Arg-rich motif, R/KXXRRXRR, which isalso conserved in all HIV-1 isolates, is required for efficienttrans activation of the HIV-1 LTR (32). The left half of the2BD has a substantial resemblance to the 1BD (Fig. 5; boxedareas). However, substitution of the Arg-rich half of the 2BDfor the 1BD substantially reduces the trans-activation poten-tial of Tatl (6% of that of wt Tatl). This level could befurther increased by inclusion of the Lys-rich half of the2BD. This observation suggests that the overall chargedensity may play a role in efficient trans activation. Theimportance of the overall charge density, in addition to thespecific sequence requirements for efficient recognition ofTAR by the Tat BD, has also been suggested by otherstudies (6, 7, 12, 16). Examination of 2BD sequences indi-cates that the 2BD consists of an Arg-rich half (indicated bythe boxed areas in Fig. 5) and a Lys-rich half (indicated by adouble underline in Fig. 5). It would be interesting todetermine whether the 2BD is a bipartite BD evolved forinteraction with a more complex TAR2 structure. It shouldbe noted that the Tat2 mutant [Tat2(99)] lacking the secondexon trans activates the HIV-2 LTR at about one-half the

B

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BASIC DOMAINS OF THE Tat PROTEINS OF HIV-1 AND HIV-2 2035

FIG. 6. Subcellular localization of Tatl, Tat2, and various mutant Tat proteins. COS-7 cells were transfected with various Tat-expressingplasmids and analyzed by indirect immunofluorescence. Panels: a, Tatl; b, Tat2; c, Tat2(99); d, Tat(99)/lBD; e, Tat(99)77; f, Tat(99)/lCD.

level of wt Tat2 (Fig. 3B). It remains to be determinedwhether the protein region encoded by exon 2 has any role inTat2-TAR2 interaction. Since exon 2 clearly is not requiredfor trans activation of LTR-1, while it is required for efficienttrans activation of LTR-2, it may be possible that the exon2-encoded protein region cooperates with the basic domainor confers a favorable structural feature for efficient recog-nition of TAR2.

ACKNOWLEDGMENTS

This work was supported by research grants AI-29541 and Al-29200.We thank J. Nelson, K.-T. Jeang, and the AIDS Research and

Reference Program for Tatl and Tat2 antibodies, M. Emerman forpROD214, and B. R. Cullen for pBC12. We also thank L. K.Venkatesh for advice and discussions.

REFERENCES1. Arya, S. K., and R. C. Gallo. 1988. Human immunodeficiency

virus type 2 long terminal repeat: analysis of regulatory eje-ments. Proc. Natl. Acad. Sci. USA 85:9753-9757.

2. Berkhout, B., A. Gatignol, A. B. Robson, and K.-T. Jeang. 1990.TAR-independent activation of the HIV-1 LTR: evidence thatTat requires specific regions of the promoter. Cell 62:757-767.

3. Berkhout, B., A. Gatignol, J. Silver, and K.-T. Jeang. 1990.Efficient trans-activation by the HIV-2 Tat protein requires aduplicated TAR RNA structure. Nucleic Acids Res. 18:1839-1846.

4. Berkhout, B., R. Silverman, and K.-T. Jeang. 1989. TAT trans-activates the human immunodeficiency virus through a nascentRNA target. Cell 59:273-282.

5. Braddock, M., A. Chambers, W. Wilson, M. P. Esnouf, S. E.Adams, A. J. Kingsman, and S. M. Kingsman. 1989. HIV-1 Tat"activates" presynthesized RNA in the nucleus. Cell 58:269-279.

6. Calnan, B. J., S. Biancalana, D. Hudson, and A. D. Frankel.1991. Analysis of arginine-rich peptides from the HIV tatprotein reveals unusual features of RNA-protein recognition.Genes Dev. 5:201-210.

7. Calnan, B. J., B. Tidor, S. Biancalana, D. Hudson, and A. D.Frankel. 1991. Arginine-mediated RNA recognition: the argi-nine fork. Science 252:1167-1171.

8. Carroll, R., L. Martarano, and D. Derse. 1991. Identification of

lentivirus Tat functional domains through generation of equineinfectious anemia virus/human immunodeficiency virus type 1tat gene chimeras. J. Virol. 65:3460-3467.

9. Clavel, F., M. Guyader, D. Guetard, M. Salle, L. Montagnier,and M. Alizon. 1986. Molecular cloning and polymorphism ofthe human immune deficiency virus type 2. Nature (London)324:691-695.

10. Cordingly, M. G., R. L. LaFemina, P. L. Callahan, J. H.Condra, V. V. Sardana, D. J. Graham, T. M. Nguyen, K.LeGrow, L. Gotlib, A. J. Schlabach, and R. J. Colonno. 1990.Sequence-specific interaction of Tat protein and Tat peptideswith the transactivation-responsive sequence element of humanimmunodeficiency virus type 1 in vitro. Proc. Natl. Acad. Sci.USA 87:8985-8989.

11. Cullen, B. R. 1990. The HIV-1 tat protein: an RNA sequence-specific processivity factor? Cell 63:655-657.

12. Delling, U., S. Roy, M. Sumner-Smith, R. Barnett, L. Reid, C. A.Rosen, and N. Sonenberg. 1991. The number of positivelycharged amino acids in the basic domain of tat is critical fortrans-activation and complex formation with TAR RNA. Proc.Natl. Acad. Sci. USA 88:6234-6238.

13. Dingwall, C., I. Ernberg, M. J. Gait, S. M. Green, S. Heaphy, J.Karn, A. D. Lowe, M. Singh, and M. A. Skinner. 1990. HIV-1 tatprotein stimulates transcription by binding to U-rich bulge in thestem of the TAR RNA structure. EMBO J. 9:4145-4153.

14. Dorn, P., L. DaSilva, L. Martarano, and D. Derse. 1990. Equineinfectious anemia virus tat: insights into the structure, function,and evolution of lentivirus trans-activator proteins. J. Virol.64:1616-1624.

15. Emerman, M., M. Guyader, L. Montagnier, D. Baltimore, andM. A. Muesing. 1987. The specificity of the human immunode-ficiency virus type 2 transactivator is different from that ofhuman immunodeficiency virus type 1. EMBO J. 6:3755-3760.

16. Endo, S.-I., S. Kubota, H. Siomi, A. Adachi, S. Oroszlan, M.Maki, and M. Hatanaka. 1989. A region of basic amino-acidcluster in HIV-1 tat protein is essential for trans-acting activityand nucleolar localization. Virus Genes 3:99-110.

17. Feng, S., and E. C. Holland. 1988. HIV-1 tat trans-activationrequires the loop sequence within tar. Nature (London) 334:165-167.

18. Fenrick, R., M. H. Malim, J. Hauber, S.-Y. Le, J. Maizel, andB. R. Cullen. 1989. Functional analysis of the Tat trans activatorof human immunodeficiency virus type 2. J. Virol. 63:5006-5012.

19. Garcia, J. A., D. Harrich, E. Soultanakis, F. Wu, R. Mitsuyasu,

VOL. 66, 1992

2036 ELANGOVAN ET AL.

and R. B. Gaynor. 1989. Human immunodeficiency virus type 1:LTR TATA and TAR region sequences required for transcrip-tional regulation. EMBO J. 8:765-778.

20. Guyader, M., M. Emerman, P. Sonigo, F. Clavel, L. Montagnier,and M. Alizon. 1987. Genome organization and transactivationof the human immunodeficiency virus type 2. Nature (London)326:662-669.

21. Hauber, J., M. H. Malim, and B. R. Cullen. 1989. Mutationalanalysis of the conserved basic domain of human immunodefi-ciency virus tat protein. J. Virol. 63:1181-1187.

22. Kammann, M., J. Laufs, J. Schell, and B. Gronenborn. 1989.Rapid insertional mutagenesis of DNA by polymerase chainreaction (PCR). Nucleic Acids Res. 17:5404.

23. Kuppuswamy, M., T. Subramanian, A. Srinivasan, and G.Chinnadurai. 1989. Multiple functional domains of Tat, thetrans-activator of HIV-1, defined by mutational analysis. Nu-cleic Acids Res. 17:3551-3561.

24. Malim, M. H., J. Hauber, R. Fenrick, and B. R. Cullen. 1988.Immunodeficiency virus rev trans-activator modulates theexpression of the viral regulatory genes. Nature (London)335:181-183.

25. Meyers, G., S. F. Josephs, J. A. Berzofsky, A. B. Rabson, T. F.Smith, and F. Wong-Staal. 1990. Human retroviruses and AIDS.Los Alamos National Laboratory, Los Alamos, N.Mex.

26. Pearson, L., J. Garcia, F. Wu, N. Modesti, J. Nelson, and R.Gaynor. 1990. A transdominant tat mutant that inhibits tat-induced gene expression from the human immunodeficiencyvirus long terminal repeat. Proc. Natl. Acad. Sci. USA 87:5079-5083.

27. Peterlin, B. M., P. A. Luciw, P. J. Barr, and M. D. Walker.

1986. Elevated levels of mRNA can account for the trans-activation of human immunodeficiency virus (HIV). Proc. Natl.Acad. Sci. USA 83:9734-9738.

28. Rosen, C. A., J. G. Sodroski, and W. A. Haseltine. 1985. Thelocation of cis-acting regulatory sequences in the human T celllymphotropic virus type III (HTLV-III/LAV) long terminalrepeat. Cell 41:813-823.

29. Roy, S., U. Delling, C.-H. Chen, C. A. Rosen, and N. Sonenberg.1990. A bulge structure in HIV-1 TAR RNA is required for Tatbinding and Tat-mediated trans-activation. Genes Dev. 4:1365-1373.

30. Selby, M. J., E. S. Bain, P. A. Luciw, and B. M. Peterlin. 1989.Structure, sequence, and position of the stem-loop in tar deter-mine transcriptional elongation by tat through the HIV-1 longterminal repeat. Genes Dev. 3:547-558.

31. Siomi, H., H. Shida, M. Maki, and M. Hatanaka. 1990. Effectsof a highly basic region of human immunodeficiency virus tatprotein on nucleolar localization. J. Virol. 64:1803-1807.

32. Subramanian, T., R. Govindarajan, and G. Chinnadurai. 1991.Heterologous basic domain substitutions in the HIV-1 Tatprotein reveal an arginine-rich motif required for trans-activa-tion. EMBO J. 10:2311-2318.

33. Subramanian, T., M. Kuppuswamy, L. Venkatesh, A. Srini-vasan, and G. Chinnadurai. 1990. Functional substitution of thebasic domain of the HIV-1 trans-activator, Tat, with the basicdomain of the functionally heterologous Rev. Virology 176:178-183.

34. Weeks, K. M., C. Ampe, S. C. Schultz, T. A. Steitz, and D. M.Crothers. 1990. Fragments of the HIV-1 Tat protein specificallybind TAR RNA. Science 249:1281-1285.

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